As one of vehicle travel control devices, there has been well known a steering device of transfer ratio varying type which has a transfer ratio varying actuator that varies a transfer ratio of rotational angle from a steering input shaft to a steering output shaft, a steering operation force alleviating actuator that generates an assist torque for alleviating the load on steering operation of a driver, and a control means that controls the operations of the transfer ratio varying actuator and the steering operation force alleviating actuator. Such a device is described, for example, in Japanese Patent Laid-Open Publication No. 2006-131141 which corresponds to an application filed by the same applicant as the present application.
As is described in Japanese Patent Laid-Open Publication No. 2006-131141, it has already been known that in a steering device of transfer ratio varying type, it is preferable to compare the operational velocity of the steering operation force alleviating actuator and the rotation angular velocity of the steering output shaft with each other and to control the operation of the steering device of transfer ratio varying type at a velocity below the operational velocity of the steering operation force alleviating actuator.
According to such a steering device of transfer ratio varying type as described in the above publication, the operational velocity of the steering device of transfer ratio varying type can be limited to a rotation angular velocity which is below the operational velocity of the steering operation force alleviating actuator, which enables to prevent the rotational speed of the steering output shaft attained by the operation of the steering device of transfer ratio varying type from exceeding the operational velocity of the steering operation force alleviating actuator. Accordingly, a deterioration of steering feeling and an increase of steering operational load can be prevented from occurring which are caused by the rotational speed of the steering output shaft exceeding the operational velocity of the steering operation force alleviating actuator.
However, if the operational velocity of the steering device of transfer ratio varying type is limited to a rotation angular velocity which is below the operational velocity of the steering operation force alleviating actuator, a transfer ratio of rotational angle from a steering input shaft to a steering output shaft is as well restricted to a small value. Therefore, in such a steering device of transfer ratio varying type as described in the above publication, a vehicle turning responsiveness to steering operation of a driver inevitably decreases.
In particular, such a problem is notable when a steering operation is conducted at a very higher speed by a driver, such as in the case of urgent steering for avoiding collision against an obstacle. The above problem is not confined to a steering device of transfer ratio varying type having a transfer ratio varying actuator which varies a transfer ratio of rotational angle from a steering input shaft to a steering output shaft. That is, the problem occurs as well when a ratio of the velocity of turning steerable wheels to the velocity of steering operation of a driver is restricted in a so-called steer-by-wire type steering control device which has a steering input means operated by a driver and an actuator for drivingly turns steerable wheels to steer, and is capable of turning steerable wheels independently of steering operation of a driver.